Hybrid integrated circuits used in automotive applications often employ thick film electronic modules which are mounted on a surface of a single layer ceramic substrate. Conductors are typically printed on the substrate to electrically interconnect the input/output terminals of the electronic modules with their respective external conductors formed on the substrate, which interconnect the conductors with an external circuit.
At times it is required to isolate these electronic modules from air, dust and moisture by hermetically enclosing them within a cover or housing that is bonded to the substrate. To do so, a suitable conductive material, such as a palladium-silver composition, is first deposited on the substrate, in that solder compositions will not adequately wet the substrate material. A suitable solder composition is then fired over the conductive material, and the cover is bonded to the substrate using a solder fellow process. Because the conductors which interconnect the input/output terminals of the electronic modules with the external conductors are printed on the surface of the substrate, the conductors are routed directly through the hermetic seal formed by the solder between the cover and the substrate. As a result, it is necessary to first fire a dielectric material over the conductors in the area where the cover is to be soldered to the substrate in order to provide electrical insulation between the conductors and the conductive material. The conductive material is then deposited, and the solder is fired over the conductive material in order to bond the cover to the substrate and, in the effected locations, the dielectric material.
A drawback to the approach described above is that the solder-conductive material-dielectric-conductor bond is not as strong as the solder-conductive material-substrate bond. Furthermore, stresses are created in the entire module structure as a result of differences in the physical properties of the substrate, conductor, dielectric, conductive material, solder and cover materials. In particular, different thermal coefficients of expansion between the individual materials tend to create stresses as a result of the soldering process during assembly and thermal cycling which occurs in service. As would be expected, such internal stresses significantly reduce the reliability of the bonds, and particularly the solder-conductive material-dielectric-conductor bond. In that the failure of any one of the bonds will also result in the loss of the hermetic seal between the cover and the substrate, such stresses may ultimately lead to the ingress of air, dust and moisture within the cover, allowing the hostile environment to attack the electronic module.
Thus, it would be desirable if an alternative method were available by which a hermetically sealed electronic module could be electrically interconnected with its external conductors without jeopardizing the integrity of the module's hermetic seal. A suitable method would not significantly complicate the processing of the electronic module, its cover or the ceramic substrate, nor have a detrimental effect on the ability to readily interconnect the module's input/output terminals to their respective conductors under high volume manufacturing conditions, such as that demanded for the automotive industry.